The heatmap reads the risk list and the assessments from other parts of the process. You cannot change this directly on the graph.
To change the titles of the axis you must update the wording within each assessment that you are plotting.
The scale will also be read from the assessment and cannot be changed directly on the graph.
Please note the colour of the font is also read from the assessment editor.
When in the List Editor, click on the Assessments button at the bottom of the screen and select the assessment that relates to the axis you wish to change. Double click on the wording you want to change in order to update it. When finished click Escape to return to the list.
Load the Matrix graph to check the change that you have made.